This invention relates generally to material separation and, more particularly, to the removal of fouling matter which forms or collects on processing equipment during separation processing, such as the separation of solids from liquid.
As separation processes generally involve the transformation of a mixture of substances into two or more products which differ from each other in composition and/or physical properties, solids-liquid separation involves the separation of two phases, e.g., solid and liquid, from a mixture. Solids-liquid separation techniques find wide use in petroleum and oil feedstock refining; chemical manufacturing, including pharmaceutical production; pollution control; and many manufacturing processes, including food (e.g., fruit and vegetable juice processing), agricultural (e.g., food grain processing) metallurgy (e.g., steel processing) and semiconductor production, for example.
A common solids-liquid separation technique involves surface filtration which is essentially a straining mechanism whereby solid particles are screened, e.g., filtered, from a solids-liquid feed stream onto a matrix element, e.g., a filter, characterized as having a controlled pore size. In such surface filtration, the flow rate of liquid through the filter decreases as solids accumulate and plug pores of the matrix.
In general, the ways in which particles interact with the pores and surface of the matrix and result in plugging of the matrix surface are not well understood. As a result, it is common to simply run filtration tests using a sample of the particular solids-containing liquid feed to be filtered and a filter matrix (i.e., test matrix) having pores small enough to produce a filtrate having suitably desired clarity. The matrix surface area required for a designed filtration system can be estimated by measuring the volume of the sample feed which passes through the test matrix before the test matrix becomes unsuitably plugged. This measured volume can then be scaled up in direct ratio to the surface area of the test matrix to calculate the surface area required of the matrix for the designed filtration system.
When a matrix element becomes undesirably plugged or clogged during use, the plugged or clogged matrix element is replaced with a new matrix element which is not so plugged or clogged or, in the alternative, the plugged or clogged matrix element is subjected to cleaning treatments which are typically periodic or cyclic in nature and by which the matrix element is suitably cleaned or "unplugged."
One approach which has or can be used for the cleaning of some objects is the use of ultrasonic wave energy. In the past, when an article or element was to be cleaned ultrasonically, such an article or element was simply immersed in a liquid medium which was ultrasonically activated to produce cavitation in the liquid medium which in turn beneficially results in removal of undesired material from the article or element being cleaned. Most sonic cleaning apparatuses basically include a bath and an electrical ultrasonic wave generator. The bath (i.e., a tank or container for holding a cleaning solution) is provided with one or more magnetostrictive or electrostrictive transducers which, when energized by means of the generator, convert electrical energy to mechanical vibrations. The high-frequency, high-energy vibrations of the transducers cause cavitation of the cleaning solution at or on the surface of the article, which in turn accelerates and aids in the removal of contaminates from the article immersed in the solution.
Such an approach in its application to plugged or clogged matrix elements, however, suffers as it requires the removal of the plugged matrix element from its associated housing for insertion in the sonic cleaning bath and subjection to cleaning by action of the high-frequency, high-energy vibrations generated in the bath, with the comparatively cleaned/unplugged matrix element subsequently being returned to its housing. Such a cleaning process involving the removal of an undesirably plugged or clogged matrix element from its housing, the subjection of such a matrix element to the action produced by ultrasonic energy and the return of a comparatively clean or unplugged matrix element to its housing severely limits the period of time which the matrix element is on-stream for the processing of solids-containing liquid feed, as well as dramatically increasing the costs associated with the cleaning of matrix elements. As a result, for many applications from the perspective of direct economic costs, it is more economical to simply discard a plugged matrix element and replace it in the system with a new matrix element. Alternatively, other "cleaning" methods may be used which methods are less manually intensive and/or involve less "downtime," i.e., time for which the matrix element and/or associated housing is off-stream and not utilized for the treatment of solids-containing liquid feed.
In practice, the typical methods for cleaning or unplugging plugged or clogged matrix elements, such as filters, involve the "backflushing" of the undesirably plugged or clogged filter. In such backflushing methods, the undesirably plugged or clogged filter is taken off-stream, e.g., the flow of solids-containing liquid feed to and through the filter is discontinued. Subsequently, a backflush liquid is used to "backflush" the filter as the direction of liquid flow through the filter is reversed, with the direction of flow of the backflush liquid through the filter being reversed from the direction of flow of the solids-containing liquid feed through the filter when the filter is being used for filtration of such feed. While such reversal of flow will typically result in the dislodgement or removal of at least some of the fouling matter from the filter, the removal of the fouling matter is usually assisted through the selection of an appropriate backflushing liquid, e.g., by the use of a backflushing liquid which is at least partially solubilizing for the fouling matter or the materials adhering the fouling matter to the filter.
In practice, should the filter become severely fouled, e.g., so that the pressure drop across the filter is greater than about two to three times the design value of the pressure drop for the filter, backflushing of the fouled filter will not, at least alone, generally effect sufficient or adequate cleaning of the filter. Consequently, when primary reliance is made on backflushing to maintain the filters at proper filtering capabilities, backflushing must generally be done at sufficiently proximate intervals so as to avoid pressure drop differentials over the filter which exceed about two to three times the pressure drop design value for the specific filter.
Thus, the search for a relatively low-cost cleaning method which permits the cleaning to be done in situ and which is effective in the removal of fouling solids which are not easily removed by common techniques has continued.
Common solids-liquid separation applications associated with modern petroleum refinery operations include coking operations wherein coke, as well as gaseous and liquid products, are produced from heavy residual oil feedstocks.
In usual coking process applications, residual oil is heated in a furnace, passed through a transfer line and discharged into either a coking drum or a fluidized coking unit. During coking the residual feedstock is thermally decomposed to a very heavy tar or pitch which further decomposes into solid coke and vapor materials. The vapors formed during decomposition are ultimately recovered from the coking zone, and solid coke is left behind.
When a delayed coking operation is utilized, the residual oil is passed into a coking drum which eventually fills with a mass of solid coke. The vapors formed in the coking drum leave the top of the drum and are passed to a fractionating column where they are separated into liquid and gaseous products. Sometimes these products are recycled with residual oil feed to the coke drum.
In delayed coking operations the residual oil feed passing into the coking drum is stopped after a predetermined period of time and routed to another drum. The first drum is then purged of vapors, cooled and opened so the solid coke material which has filled the drum can be removed by drilling or other means.
In fluidized coking, a residual oil feed contacts a previously produced, hot fluidized bed of coke particles and is converted to additional coke material and lighter hydrocarbons. The coke in the fluidized bed is heated through external means which include either a gasification zone, where a part of the fluidized coke produced from the residual oil feed is burned with oxygen, or through heat exchange with a combustor.
In either type of coking operation, the refiner generally aims to minimize coke production and maximize liquid products from a residual oil feed, since liquid products are more easily converted into gasoline or other products of higher value than solid coke.
Additional solids-liquid separation applications associated with modern petroleum refinery operations include gasoline and diesel oil filtration, decanted oil filtration, pyrolized and coke oven gas filtration, filtration operation prior to processing in fixed bed reactor units associated with many operations, as well as flue gas solids removal and other environmental applications.